1. Field of the Invention
The present invention relates to a method for producing tyres which are different from each other within one production plant.
2. Description of the Related Art
A tyre for vehicle wheels normally comprises a carcass structure, essentially consisting of one or more carcass plies shaped in an essentially toroidal configuration and having their axially opposing lateral edges engaged with corresponding annular reinforcing structures incorporating circumferentially inextensible inserts usually called “bead wires”. Each annular reinforcing structure is incorporated in what is known as a “bead” formed along an inner circumferential edge of the tyre for fixing the tyre to a corresponding mounting rim.
A belt structure, comprising one or more strips of belt in the shape of a closed loop, essentially consisting of textile or metal cords suitably orientated with respect to each other and with respect to the cords belonging to the adjacent carcass plies, is applied to the carcass structure in a radially external position.
A tread band, normally consisting of a strip of elastomeric material of suitable thickness, is also applied to the belt structure in a radially external position.
It should be noted that, for the purposes of the present description, the term “elastomeric material” denotes the rubber mixture in its entirety, in other words the whole material formed by at least one polymer base suitably amalgamated with reinforcing fillers, and/or process additives of various types.
A pair of sidewalls, each of which covers a lateral portion of the tyre lying between what is called a shoulder area, located near the corresponding lateral edge of the tread band, and the corresponding bead, is applied to the opposite sides of the tyre.
Given the above, it should be noted that each type of tyre is essentially distinguished from the others by a set of chemical and physical, structural, dimensional and appearance characteristics.
The chemical and physical characteristics essentially relate to the type and composition of the materials, and particularly to the recipes of the various mixtures used in the production of the elastomeric materials. The structural characteristics essentially define the number and type of the structural components present in the tyre, and their positioning with respect to each other in the structure of the tyre. The dimensional characteristics relate to the geometrical measurements and to the cross-sectional profile of the tyre (external diameter, maximum chord or width, sidewall height and their ratio, in other words the section ratio) and will be indicated simply as “specification” hereafter. The appearance characteristics consist of the design on the rolling surface of the tread, the ornamental patterns and the various pieces of wording or distinctive signs reproduced on the tyre, for example on the sidewalls of the tyre, and will be indicated as a whole as “tread design” in the remainder of the present description.
The conventional production processes essentially comprise four distinct stages in the manufacture of tyres:    a) preparation of the mixtures,    b) production of the individual structural components,    c) assembly of the different structural components in succession, to produce a crude tyre on a drum or other suitable support,    d) vulcanization of the crude tyre with simultaneous stamping of the tread design on the external surface of the tyre.
For the purposes of the present invention, “type of tyre” denotes a tyre having a given specification, given structural components of which it consists, and a given tread design.
In an effort to reduce production costs, technological development has been basically orientated towards the search for technical solutions which would lead to the production of increasingly fast and reliable machinery, in such a way as to minimize the time required to produce each tyre, while maintaining or improving the quality of the finished product.
Thus, plants with high production capacity in terms of pieces produced per unit of time have been produced, using tyre manufacturing machinery which has reduced options for modification (or in other words, is capable of producing only a limited range of types of tyre), but which maximize the serial production of tyres having identical structural characteristics. Purely by way of example, in the most up-to-date plants the output can be up to approximately two carcasss per minute, and the average batch output in one month of operation for each article (type of tyre) can be 3200 pieces, with an article-changeover time of 375 minutes.
Attempts have also been made to reduce or eliminate the storage of the semi-finished products present between one and another of the four process stages listed above, in such a way as to minimize the costs and problems involved whenever the type of tyre in production has to be changed. For example, the document EP 922561 proposes a method for controlling tyre production, in which, in order to reduce or eliminate both the crude tyre storage time and the number of crude tyres being stored, a complex vulcanizing unit is provided, with a number of moulds suitable for constantly absorbing the output of the complex tyre manufacturing unit. The production of tyres of different types, particularly those having different specifications, is achieved by replacing and/or adapting from time to time the machinery provided in the complex tyre manufacturing unit, in conjunction with the replacement of the moulds in the complex vulcanizing unit.
The applicant has found that, in all cases, the production of the tyres entails costs which increase with the variety of types of tyre to be produced: in particular, it is necessary to intervene in the processes and/or mixture production plants to permit the production of components with new and different physical and chemical characteristics and/or in the production plants of the individual structural components to change the specification of the tyres being produced. It is also necessary to change the operating sequence (different assembly method) and/or the equipment and adjustment of the manufacturing machinery whenever a change is made in the structure and/or the specification of the tyre to be produced. Finally, it is necessary to have at least one vulcanizing mould for each different tread design-specification pair.
All of the above entails continuing costs for the purchase of moulds with different specifications and different tread designs, and of different equipment, costs for introducing the latter, losses of output due to machine downtime (a change of process or equipment generally causes machine downtime), and waste of material. For example, in the case of continuous production of components, machine downtime of downstream plants and/or a change in the characteristics of the components generates excess production which has to be rejected, since it is impossible to re-use it.
Given these circumstances, in the applicant's opinion the production of a large number of types of tyre in a single plant is generally undesirable, particularly if the objective of minimizing costs is to be pursued. In fact, this objective is incompatible with a frequent change of equipment and production processes. When production processes of the conventional type are used, the applicant has observed that, where the volume of sales of each individual type is sufficiently high, the number of the production plants can be multiplied in such a way as to make it possible to produce a different type continuously in each plant, thus minimizing the aforesaid disadvantages. On the other hand, where the volumes of sales forecast for specific types are not particularly high, for example on an annual basis, it is also possible in each case to carry out the whole production for at least one year immediately and continuously, to contain the production costs for these types. This system may, however, affect the quality of the sold product, and tends to increase storage costs, since the products remain in stock for a long period. The risk associated with sales also increases, for example as a result of unforeseen rapid obsolescence of the product, and there is an increase in the financial costs of capital tied up in the stocks of the product and in the installation of the moulds which are to be used only for the restricted period necessary to complete the production of the forecast reduced volume.
In order to tackle these problems, the applicant has already developed a production method in which each series of tyres identical to each other as regards production is broken down into daily lots, each comprising a quantity of tyres sufficient to cover the daily output of one mould. In this way the production of tyres having different specifications and/or different constructional characteristics is optimized by eliminating the storage of large quantities of crude and vulcanized tyres. This method is described in European patent application EP 875364 in the name of the present applicant.
In a tyre production plant, the stage of vulcanization of the tyre is carried out in a period which is essentially identical for ranges of all the types of tyres, but on the other hand the tyre manufacturing time differs considerably according to the type of tyre to be produced. Additionally, the application of even a single component takes different lengths of time for different types of tyre.
This impedes a frequent change of type within the plant described above, since the creation of waiting times for the vulcanization stage would occur whenever a tyre to be vulcanized belonged to a different type from that preceding it in the crude tyre processing sequence.
Moreover, a frequent change of type of tyre within one processing batch also entails a frequent change of the equipment for making the different types, thus further increasing the waiting times.
For the purposes of the present invention, the term “serial processing plant” denotes a plant in which the individual stages of processing of the tyre are carried out in a fixed sequence, in other words in which each tyre processing stage starts immediately after the preceding stage has ended.
The applicant has observed that, in a serial processing plant, the total production process time is dependent on the slowest processing stage.
For the purposes of the present invention, the term “critical processing period” denotes a processing period in which no changes are planned in the equipment during the tyre processing sequence.